Material-discerning proximity sensing

ABSTRACT

A material-discerning proximity sensor is arranged to include an antenna that is arranged to radiate a radio-frequency signal. A capacitive sensor is arranged to detect a change in capacitance of the capacitive sensor and to receive the radio-frequency signal. An electrical quantity sensor is arranged to detect a change of the received radio-frequency signal.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.13/533,955 filed Jun. 26, 2012, which claims priority to U.S.Provisional Application No. 61/641,972, filed May 3, 2012, the entirecontent of each of the applications listed above being incorporated byreference herein.

BACKGROUND

Contemporary proximity sensing techniques are used to determine whether(and at what distance) an object has entered into a range of a proximitysensor of an autonomous electronic system. For example, a capacitiveelectrode is able to discern the proximal presence of an object and inresponse activate a function of the autonomous electronic system.However, conventional sensors often have difficulty determining thenature of the proximal object and/or the material(s) that comprise theobject. Accordingly, false positive proximal detections often occur forcertain objects that enter the range of the proximity sensor.

SUMMARY

The problems noted above are solved in large part by material-discerningproximity sensing that includes an antenna that is arranged to radiate aradio-frequency signal. A capacitive sensor is arranged to detect achange in capacitance of the capacitive sensor and to receive theradio-frequency signal. An electrical quantity sensor is arranged todetect a change of the received radio-frequency signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an illustrative computing device in accordance withembodiments of the disclosure;

FIG. 2 is a schematic diagram illustrating a material-discerningproximity sensing system in accordance with embodiments of thedisclosure;

FIG. 3 is a schematic diagram illustrating radiated lobes of amaterial-discerning proximity sensing sensor in accordance withembodiments of the disclosure; and

FIG. 4 is a flow diagram illustrating material-discerning proximitysensing in accordance with embodiments of the disclosure.

DETAILED DESCRIPTION

The following discussion is directed to various embodiments of theinvention. Although one or more of these embodiments may be preferred,the embodiments disclosed should not be interpreted, or otherwise used,as limiting the scope of the disclosure, including the claims. Inaddition, one skilled in the art will understand that the followingdescription has broad application, and the discussion of any embodimentis meant only to be exemplary of that embodiment, and not intended tointimate that the scope of the disclosure, including the claims, islimited to that embodiment.

Certain terms are used throughout the following description—andclaims—to refer to particular system components. As one skilled in theart will appreciate, various names may be used to refer to a component.Accordingly, distinctions are not necessarily made herein betweencomponents that differ in name but not function. In the followingdiscussion and in the claims, the terms “including” and “comprising” areused in an open-ended fashion, and thus are to be interpreted to mean“including, but not limited to . . . . ” Also, the terms “coupled to” or“couples with” (and the like) are intended to describe either anindirect or direct electrical (including electromagnetic) connection.Thus, if a first device couples to a second device, that connection canbe made through a direct electrical (including electromagnetic)connection, or through an indirect electrical connection via otherdevices and connections.

FIG. 1 shows an illustrative computing device 100 in accordance withembodiments of the disclosure. For example, the computing device 100 is,or is incorporated into, a mobile communication device 129, such as amobile phone, a personal digital assistant (e.g., a BLACKBERRY® device),a personal computer, automotive electronics, projection (and/ormedia-playback) unit, or any other type of electronic system.

In some embodiments, the computing device 100 comprises a megacell or asystem-on-chip (SoC) which includes control logic such as a CPU 112(Central Processing Unit), a storage 114 (e.g., random access memory(RAM)) and tester 110. The CPU 112 can be, for example, a CISC-type(Complex Instruction Set Computer) CPU, RISC-type CPU (ReducedInstruction Set Computer), or a digital signal processor (DSP). Thestorage 114 (which can be memory such as on-processor cache,off-processor cache, RAM, flash memory, or disk storage) stores one ormore software applications 130 (e.g., embedded applications) that, whenexecuted by the CPU 112, perform any suitable function associated withthe computing device 100. The CPU 112 can include (or be coupled to) aproximity determination 134 unit, which includes various componentsarranged in a common (or separate) substrate as disclosed herein below.

The tester 110 is a diagnostic system and comprises logic (embodied atleast partially in hardware) that supports monitoring, testing, anddebugging of the computing device 100 executing the software application130. For example, the tester 110 can be used to emulate one or moredefective or unavailable components of the computing device 100 to allowverification of how the component(s), were it actually present on thecomputing device 100, would perform in various situations (e.g., how thecomponents would interact with the software application 130). In thisway, the software application 130 can be debugged in an environmentwhich resembles post-production operation.

The CPU 112 comprises memory and logic that store information frequentlyaccessed from the storage 114. The computing device 100 is oftencontrolled by a user using a UI (user interface) 116, which providesoutput to and receives input from the user during the execution thesoftware application 130. The output is provided using the display 118,indicator lights, a speaker, vibrations, image projector 132, and thelike. The input is received using audio and/or video inputs (using, forexample, voice or image recognition), and mechanical devices such askeypads, switches, proximity detectors, and the like. The CPU 112 andtester 110 is coupled to I/O (Input-Output) port 128, which provides aninterface (that is configured to receive input from (and/or provideoutput to) peripherals and/or computing devices 131, including tangiblemedia (such as flash memory) and/or cabled or wireless media. These andother input and output devices are selectively coupled to the computingdevice 100 by external devices using wireless or cabled connections.

As disclosed herein, material-discerning proximity sensing techniquesallow an autonomous electronic system to more accurately determine thesubstance of a proximal object by evaluating characteristics ofmaterials that are included by the proximal object. Discernment of thecharacteristics of materials included by the proximal object is used toreduce (and/or eliminate) problems that are associated with falsepositive proximal detections.

FIG. 2 is a schematic diagram illustrating a material-discerningproximity sensing system in accordance with embodiments of the presentdisclosure. System 200 is an example autonomous electronic system thatis arranged to perform material-discerning proximity sensing. System 200includes a processor 210, a low pass filter 212, a, common matchingnetwork 214, an antenna 220, a capacitive proximity sensor 230, a filter232, and an analog to digital (ADC) converter 240.

Processor 210 is a processor such as CPU 112 that is generally arrangedto control functions of system 200 in response to the closeness of thematerial characteristics of proximal object, such as a human finger.Processor 210 generates and/or controls a single- or dual-endedradio-frequency signal that is adapted to drive antenna 220. Theradio-frequency signal is a repetitive wave function, which can be asine wave, a square wave, or other waveforms suitable for drivingantenna 220. For example, a square wave signal can be filtered by lowpass filter 212 to pass a fundamental frequency (at a frequency such as13.56 megahertz). Common matching network 214 is arranged to balance theimpedance of the feed lines to the antenna 220 with the characteristicimpedance of the antenna 220 itself.

In the illustrated embodiment, antenna 220 is arranged as a coil whereinthe antenna, when energized, has an electrical core that extends througha portion of the surface of the capacitive proximity sensor 230. Thecoil of antenna 220 can be arranged as, for example, a series ofconductive traces that progressively wind or loop (one or more times)around an inner portion of the capacitive sensor. When the conductivetraces are arranged in a rectilinear fashion, each segment (or group ofsegments) is shorter (or longer, depending on a direction in which thesegments are traversed) such that the segments progressively “spiral”inwards to (or outwards from, depending on the direction in which thesegments are traversed) the capacitive proximity sensor 230. (In analternate embodiment, the conductive traces can also be arranged usingcurved traces to form a curved spiral that is wound around thecapacitive proximity sensor 230.)

The conductive traces have a length “l₁” that is longer than the length“l₂” of the capacitive proximity sensor 230 and a width “w₁” that iswider than the width “w₂” of the capacitive proximity sensor 230. Eachsegment of the conductive traces is separated (e.g., by a dielectric)from an adjacent segment of the conductive traces by a distance “d₁” andhas a width of “d₀.” Thus the conductive traces are arranged to bemutually inductive and form an electrical field in response to anapplied (e.g., time invariant) radio-frequency signal (coupled from theprocessor 210, for example) being coupled to opposite end of theconductive traces. The conductive traces need not lie in the same planeas the capacitive proximity sensor 230, need not surround the capacitivesensor, and even have various shapes, but are arranged to electricallyinteract with the capacitive proximity sensor 230.

The total length of the conductive traces (as well as the number of“turns,” the separation between adjacent segments, and width and lengthof each of the segments) can be selected in accordance with a fractionof the wavelength of the radio-frequency signal (e.g., tone and/orcarrier wave) coupled to the antenna 220. The range “r” anddirectionality of the radiated electric field are also affected by theshape, proportions, trace width, distance between traces, total innerperimeter of the conductive traces and total outer perimeter of theconductive traces.

The electric field is illustrated using field lines {right arrow over(E)}, where field lines {right arrow over (E₁)} are generated inassociation with a capacitive sensor mode and field lines {right arrowover (E₂)} are generated in association with a radio frequency/materialdiscernment mode. The field lines {right arrow over (E₁)} illustrate theelectric field coupled between the object (to be detected) and theassociated portions of the capacitive proximity sensor 230 and {rightarrow over (E₂)} illustrate the electric field coupled between coil ofantenna 220 and the associated portions of the capacitive proximitysensor 230. An upper and lower main lobe of the electric and magneticfields can be used to detect the object (to be detected) having range“r.” The electric field is also associated with magnetic components{right arrow over (E)}. Both conductive and non-conductive objects canpossibly impact the strength of the {right arrow over (E)} field of thecoil antenna, which is associated with field lines {right arrow over(E₂)}. Thus, in the material discernment mode, as the {right arrow over(E)} field is impacted, the {right arrow over (E₂)} field is thus alsoimpacted, which causes a change in the amplitude on the associatedportions of the capacitive proximity sensor 230 (due to the changes inthe {right arrow over (E₂)} field), for example.

Thus, the antenna 220 is arranged as a coil that, when energized,generates an electrical field having upper lobes and lower lobes, with amain upper lobe and main lower lobe defining an axis that extendsthrough a portion of the surface of the capacitive proximity sensor 230(as discussed below with respect to FIG. 3). When viewed as anorthogonal projection using an axis of projection (as viewed from above,for example) that is not parallel to a portion of the surface of thecapacitive proximity sensor 230, the traces appear to surround thecapacitive proximity sensor 230.

For ease of commercialization, the antenna that is arranged to radiate aradio-frequency signal can be driven using a transmit output power belowthe FCC threshold requiring certification (e.g., for a frequency bandthat includes the frequency of the radio-frequency signal coupled to theantenna 220).

Capacitive proximity sensor 230 is, for example, a copper fill padhaving an area determined as the (multiplication) product of the length“l₂” and the width “w₂” of the capacitive proximity sensor 230. (Theaspect ratio of a, for example, rectangular capacitive proximity sensor230 can vary and the area thereof, for example, can be larger or smallerthan the area of a human finger.) The copper fill pad of capacitiveproximity sensor 230 is formed on a fixed substrate such as a printedcircuit board (PCB) or formed on a flexible substrate such as a flexiblePCB. As discussed above, in an embodiment coil antenna is arrangedaround the perimeter of the capacitive proximity sensor 230.

The capacitive proximity sensor 230 is arranged to discern the proximalpresence of an object by detecting a change in capacitance of thecapacitive sensor. The capacitive proximity sensor 230 is also used as asensor for the discernment of the material of the proximal object bysensing the disruption (and the degree of disruption) of the electricfield produced by antenna 220. Thus the capacitive proximity sensor 230is used to make two differing types of measurements. In an embodiment,the measurements are time-multiplexed where the types of measurementsare alternated.

System 200 uses the capacitive proximity pad in conjunction with anelectrical quantity sensor such as an ADC (analog-to-digital converter)to measure the level at the applied frequency of the electrical fieldcoupled to the capacitive proximity sensor 230 from the surrounding coilantenna 220. A function of the electrical quantity sensor is to quantify(in units of time, resistance, capacitance, and the like) a detectedelectrical property that is associated with the capacitive proximitypad. As various objects move into the field of the antenna, they impactand interfere with the tuning and efficiency of the antenna 220 and thecommon matching network 214 (which can be matched to the antenna 220).Objects in the field that are conductive affect characteristics of themagnetic field (and the concomitant electric field) output by theantenna 220 to a substantially greater degree than non-conductiveobjects. One characteristic of the characteristics of the electric fieldthat is changed is manifested as a change in amplitude of theradio-frequency signal used to generate the electric and magnetic fieldscoupled to the capacitive proximity pad.

The change in amplitude of radio-frequency signal can be detected byusing measurements performed by the ADC 240. The ADC 240 forwards themeasurements as data to be used by software and/or firmware of theprocessor 210. Filter 232 is optionally employed to filter the receivedradio-frequency signal to prevent and/or reduce aliasing of the sampledradio-frequency signal by the ADC 240.

In an embodiment, a low-speed ADC 240 is used to minimize powerconsumption, complexity, and layout area. With the low-speed ADC 240,under-sampling and aliasing are intentionally used in a manner thatallows for signal energy at the ADC 240 input to be detected whileproviding increased immunity to noise.

Without external filtering (to maintain a low cost, for example), theamplitude of the received radio signal frequency can still be measuredby the ADC 240 regardless of degree of aliasing caused by under-sampling(even given a large disparity in sampling rate and Nyquist rates withregards to the frequency of the radio-frequency signal). The capacitiveproximity sensor 230 that is under-sampled by the ADC 240 thuseffectively operates using a broadband input.

The total energy determined by the under-sampled ADC 240 input isdetermined by, for example, summing the magnitude of the samples of thecapacitive proximity sensor 230 (as affected by the electric field) overa selected time period (e.g., a tenth of a second) in which toaccumulate samples. (In an alternate embodiment, a software envelopedetector can be arranged to determine the total energy.) Thus; anunperturbed electric field, the presence of a non-conductive objectwithin the electric field, and the presence of noise content do notsubstantially affect the baseline level of energy at the ADC 240 input.

The amplitude of the sampled signal (even without the interveningpresence of filter 232) is not substantially incorrectly measured by theADC 240 when under-sampling the capacitive proximity sensor 230. The ADC240 is able to substantially correctly measure the energy coupled to thecapacitive proximity sensor 230 because the presence of a proximalconductive object (within range of the electric field) both lowers theenergy (signal amplitude as determined by accumulating samples over aselected time period) at the input of the ADC 240, and also tends toshield the system 200 from external noise sources. Accordingly,under-sampling by the ADC 240 provides for increased noise immunity forthe system, while also allowing the use of a relatively simple (e.g.,low cost) broadband ADC 240 to measure the capacitive proximity sensor230.

In other embodiments, more complex ADCs, comparators, sample and holdcircuits, or other common peripherals or other various types of voltagesensors may be used to detect a change in amplitude of radio-frequencysignal coupled to capacitive proximity sensor 230. The detected changein amplitude of radio-frequency signal coupled to capacitive proximitysensor 230 can be detected by accumulating samples over a selected timeperiod using an electrical quantity sensor.

In an embodiment RFID (radio-frequency identification) signal generatoris arranged to couple an RFID signal to the antenna 220 such that theantenna is used to radiate an RFID radio-frequency signal. Using theantenna 220 to radiate the RFID radio-frequency signal allows the system200 design to be more compact as it obviates the need to have an antennadedicated solely for radiating the RFID signal.

Likewise, the capacitive proximity sensor 230 is arranged to receive theRFID radio-frequency signal. Using the capacitive proximity sensor 230is arranged to receive the RFID radio-frequency signal allows the system200 design to be more compact by obviating the need to have a receivingantenna solely dedicated for receiving the RFID radio-frequency signal.

Similarly, the ADC 240 is arranged to sample the RFID radio-frequencysignal received by the capacitive sensor and to output and to transmitthe samples to the processor 210 to provide an RFID capability forsystem 200. Using the ADC 240 to sample the RFID radio-frequency signalallows the system 200 design to be more compact by sharing the use ofthe ADC for reading the RFID radio-frequency signal, reading thereceived radio-frequency signal, and measuring a capacitance of thecapacitive proximity sensor 230, for example. The readings of thereceived radio-frequency signal, the received RFID radio-frequencysignal, and the capacitance of the capacitive sensor can betime-multiplexed when transmitted to the processor 210, for example.

The selected time periods for reading of the received radio-frequencysignal, the received RFID radio-frequency signal, and the capacitance ofthe capacitive sensor can vary in accordance with the selected readingfunction. For example, the time period for reading the RFIDradio-frequency signal can be selected in accordance with the RFIDprotocols. Likewise, the time period for measuring the capacitance ofthe capacitive proximity sensor 230 can be selected in accordance with atime interval suitable for determining an RC (resistive-capacitive)time-constant associated with an implementation of the capacitiveproximity sensor 230.

Similarly, the time period for readings of the received radio-frequencysignal can be selected in accordance with a time interval suitable fordetermining the movement of a proximal object within the electric field.For a human finger moving within range “r” of a lobe of the magnetic andelectric field, a selected time interval for accumulating samples can beselected to determine the velocity of the finger moving through a lobeof the electric field.

FIG. 3 is a schematic diagram illustrating radiated lobes of amaterial-discerning proximity sensing sensor in accordance withembodiments of the disclosure. As shown in FIG. 2, antenna 220 isarranged as a coil that, when energized, generates an electrical fieldin response to a radio-frequency signal being coupled the antenna 220.In FIG. 3, the electric field 300 is illustrated as a main “lobe” of thegenerated having an upper lobe 320 and a lower lobe 330, the upper lobe320 and lower lobe 330 defining an axis that extends through a portionof the surface of the (flexible) capacitive sensor 310. The upper lobe320 and the lower lobe 330 are illustrated as geometric shapes for thesake of simplicity. In various embodiments the shape of the electricfield varies in accordance with the various shapes and arrangements ofthe antenna 220 and the capacitive sensor 310.

FIG. 4 is a flow diagram illustrating a material-discerning proximitysensing in accordance with embodiments of the disclosure. The programflow illustrated herein is exemplary, and thus various operations withinthe program flow can be performed in an order that is not necessarilythe same as the program flow illustrated herein. Program flow begins atnode 402 and proceeds to operation 410.

In operation 410, a change in the capacitance of the capacitive sensoris detected. The capacitive change is detected using any suitable methodincluding, for example, measuring an RC time-constant that is associatedwith the capacitive sensor. As discussed above, the change incapacitance can detect the proximity of an object, but substantiallyfails to discern the material that comprises the object. Program flowproceeds to operation 412.

In operation 412, a determination is made whether a change incapacitance has been detected. If a change in capacitance has notoccurred, program flow proceeds to operation 410. If a change incapacitance has occurred, program flow proceeds to operation 420.

In operation 420, a radio-frequency signal is radiated by an antennathat is substantially arranged around a capacitive sensor. The antennais substantially arranged around the capacitive sensor when the radiatedradio-frequency signal induces a voltage in the capacitive sensor.Program flow proceeds to operation 420.

In operation 430, the capacitive sensor receives the radiatedradio-frequency signal. A baseline measurement (such as when there is noobject in the proximity of the capacitive sensor) of the magnitude ofthe received radio-frequency signal. The baseline measurement can bemade by under-sampling (e.g., below Nyquist rates) the receivedradio-frequency signal using an ADC as described above to detect anenergy level of the received radio-frequency signal received over aselected time period. The under-sampling also increases the relativeamount of noise immunity of the system used to perform thematerial-discerning proximity sensing. The noise is typically generatedexternally to the system, although noise generated by the system is alsopossible. Program flow proceeds to operation 440.

In operation 440, a change in the received radio-frequency signal isdetected. The change in the received radio-frequency signal is detectedby measuring the magnitude of the received radio-frequency signal (usingthe under-sampling ADC, for example). Program flow proceeds to operation450.

In operation 450, the detected changes in the received radio-frequencysignal are monitored. The detected changes in the receivedradio-frequency signal are monitored by comparing the measured magnitudewith the baseline measurement to determine the degree of the detectedchange. The change in the received radio-frequency signal can also bedetected by measuring the magnitude of the received radio-frequencysignal and comparing the measured magnitude with a predeterminedthreshold to determine the degree of detected change. The change in thereceived radio-frequency signal can also be detected by measuring themagnitude of the received radio-frequency signal and comparing themeasured magnitude with a list of one or more thresholds that comparewith predetermined thresholds that each correspond to a type of materialof an object (such as a human finger) that would be used to make a validproximity detection. Program flow proceeds to operation 460.

In operation 460, a determination is made whether a valid proximitydetection has occurred. Comparison of the measured magnitude of thereceived radio-frequency signal with the predetermined thresholdsprovides an indication of the material that comprises a proximal object(e.g., that causes the detected change in capacitance). The measuredmagnitude is in direct proportion to the conductivity of the proximalobject. Thus, discernment of a characteristic of the material thatcomprises the proximal object increases the likelihood of a validdetection. If a valid proximity detection has not occurred, program flowproceeds to operation 430. If a valid proximity detection has occurred,program flow proceeds to operation 470.

In operation 470, a valid detection signal is output. The validdetection signal is output in response to the determination of a validdetection. The output valid detection signal is used by a processingsystem to perform an action in response to the valid detection of aproximate object (detected in response to a press of a human finger, forexample). The action performed can be any action performable by a systemsuch as accepting a security code, selection of an elevator control,dispensing a selected product from a machine, and the like. Program flowproceeds to node 490 and terminates.

The various embodiments described above are provided by way ofillustration only and should not be construed to limit the claimsattached hereto. Those skilled in the art will readily recognize variousmodifications and changes that could be made without following theexample embodiments and applications illustrated and described herein,and without departing from the true spirit and scope of the followingclaims.

What is claimed is:
 1. A material-discerning proximity sensing device,comprising an antenna that is arranged to radiate a radio-frequencysignal; a capacitive sensor that is arranged to detect a change incapacitance of the capacitive sensor and that is arranged to receive theradio-frequency signal; and an electrical quantity sensor that isarranged to detect a change of the received radio-frequency signal. 2.The device of claim 1, further comprising a matched network that isarranged to balance transmission characteristics of the antenna and tolow pass filter a signal used to drive the antenna.
 3. The device ofclaim 1, wherein the electrical quantity sensor comprises an analog todigital converter is arranged to quantify the detected change of thereceived radio-frequency signal.
 4. The device of claim 1, wherein theelectrical quantity sensor comprises an analog to digital converter isarranged to quantify the detected change of the received radio-frequencysignal using a sampling rate that is less-than-or-equal-to at leasttwice the frequency of a carrier wave of the transmitted radio-frequencysignal.
 5. The device of claim 4, wherein the analog to digitalconverter is arranged to quantify the detected change of the capacitanceof the capacitive sensor.
 6. The device of claim 5, wherein the analogto digital converter is arranged to output time-multiplexed readings ofthe received radio-frequency signal and the capacitance of thecapacitive sensor.
 7. The device of claim 1, wherein the capacitivesensor is conductive material arranged having a surface.
 8. The deviceof claim 7, wherein the surface of the capacitive sensor is planar. 9.The device of claim 7, wherein the antenna is arranged as a coil that,when energized, has an electrical core that extends through a portion ofthe surface of the capacitive sensor.
 10. The device of claim 7, whereinthe antenna is arranged as arranged as series of conductive traces on asubstrate that surround an inner portion of the capacitive sensor whenviewed as a two-dimensional projection using an axis of projection thatis not parallel to a portion of the surface of the capacitive sensor.11. The device of claim 7, wherein the antenna is arranged as a coilthat, when energized, generates an electrical and magnetic field havingan upper lobe and a lower lobe, the upper lobe and lower lobe definingan axis that extends through a portion of the surface of the capacitivesensor.
 12. The device of claim 11, an antenna that is arranged toradiate a radio-frequency signal using a transmit output power below theFCC threshold requiring certification.
 13. The device of claim 12,comprising an RFID (radio-frequency identification) signal generatorthat is arranged to couple an RFID signal to the antenna, wherein theantenna that is arranged to radiate an RFID radio-frequency signal,wherein the capacitive sensor is arranged to receive the RFIDradio-frequency signal, and wherein the analog to digital converter isarranged to output time-multiplexed readings of the receivedradio-frequency signal, the received RFID radio-frequency signal, andthe capacitance of the capacitive sensor.
 14. A material-discerningproximity sensing system, comprising: an antenna that is arranged toradiate a radio-frequency signal; a capacitive sensor that is arrangedto detect a change in capacitance of the capacitive sensor and that isarranged to receive the radio-frequency signal; a electrical quantitysensor that is arranged to detect a change of the receivedradio-frequency signal; and a processor that is arranged to monitordetected changes of the received radio-frequency signal and in responseto the detected changes of the received radio-frequency signal determinewhether a valid proximity detection has occurred.
 15. The system ofclaim 14, wherein the processor is arranged to monitor detected changesin capacitance of the capacitive sensor, and in response to the detectedchanges in capacitance of a capacitive sensor and the detected changesof the received radio-frequency signal determine whether a validproximity detection has occurred.
 16. The system of claim 14, whereinthe determination whether a valid proximity detection has occurred ismade in response to a comparison of the detected changes with apredetermined threshold.
 17. A method for material-discerning proximitysensing, comprising: radiating a radio-frequency signal using anantenna; detecting a change in capacitance of a capacitive sensor;receiving the radio-frequency signal using the capacitive sensor;detecting a change of the received radio-frequency signal; monitoringthe detected changes of the received radio-frequency signal; anddetermining whether a valid proximity detection has occurred in responseto the detected changes of the received radio-frequency signal.
 18. Themethod of claim 17, comprising: monitoring detected changes incapacitance of the capacitive sensor; and determining whether a validproximity detection has occurred in response to the detected changes incapacitance of the capacitive sensor and the detected changes of thereceived radio-frequency signal.
 19. The method of claim 18, wherein thedetermination whether a valid proximity detection has occurred is madein response to a comparison of the detected changes with a predeterminedthreshold.
 20. The method of claim 17, comprising using an analog todigital converter to quantify detected changes of the receivedradio-frequency signal using a sampling rate that isless-than-or-equal-to at least twice the frequency of a carrier wave ofthe transmitted radio-frequency signal.